Builder delaying Registation of flat

I am looking for advice regarding a serious issue I am facing. I have purchased a ready to move in flat in December 2015 directly from builder for which I paid complete amount ( 70% financed from HDFC Bank) in Faridabad. I am following up for registration of flat and builder is delaying it giving me next month's timeline everytime. Later I came to know that the builder pledged some units as a security from some other bank as a part of commercial loan. My unit is also a part of those. When I spoke to builder office first they denied but later accepted that they are waiting for NOC from the bank from which they have taken loan and once they get NOC , they will do registration of my flat on my name. I am surprised that first of all how the builder can cheat me like this. Also HDFC from which I took home loan they must have checked the title is clear or not , then how this thing was not captured in their investigation? Can I go through some legal route and if yes then how? Please suggest. Thanks